In roll forming apparatus, a flat sheet of malleable material, such as sheet metal, enters the apparatus at the entry end, is formed by a plurality of rollers, and exits the apparatus as a shaped member at the discharge end. A portion of the material passes through the apparatus without any forming. For example, in apparatus for forming a rain gutter with a trough having a flat bottom, generally the trough bottom at any point along the length of the apparatus is parallel to the flat sheet at the entry end and passes through the apparatus without being formed. The path along which the material passes without being formed is a surface called the pass line.
The pass line is defined by the forming rollers. The pass line extends longitudinally in the direction that material travels through the roll forming apparatus and laterally parallel to the axis of the rollers. The pass line may be a planar or may be a curved surface. U.S. Pat. No. 3,529,461 to Knudson discloses a pass line that follows a parabolic section along the length of the roll forming apparatus.
In prior known roll forming applications, powered drive rollers and powered forming rollers apply power at the material pass line. Powered forming rollers have contours to form or shape material with diameters that vary from the nominal pass line diameter. Generally, the nominal throughput speed of the roll forming apparatus is a function of drive roller rotational speed and diameter at the pass line. The actual linear speeds of the rollers vary where they contact the material due to changes in diameter away from pass line.
Forming that takes place above or below the pass line is often accomplished using idled forming rollers rather than powered forming rollers to avoid the speed variations that occur in powered forming rollers as diameters vary away from nominal pass line. However, forming away from the pass line without power also induces a great deal of drag on the profile. This drag typically leads to profile distortion and unbalanced residual forces in the finished profile.
In other prior known roll forming apparatus the forming of complex shapes is performed at or near the pass line. U.S. Pat. No. 4,899,566 to Knudson discloses apparatus to form ogee type rain gutter. The multiple variations from the pass line of each set of rollers for such apparatus must be accounted for in the roller design to prevent distortion and residual stress and the design is therefore complex.
Errors in the design of such complex rollers can create residual stress in the product, leading to warped output from the roll forming apparatus. Often such errors can only be corrected by fabricating and installing new rollers.